Torsion oscillators are typically driven by electrical signals applied at the resonant frequency of a body mounted between torsion members. This invention addresses the stabilization of torsion oscillators as their resonant frequency varies.
Torsion oscillators are known, although not widely employed. U.S. Pat. No. 4,762,994 to Byerly et al., U.S. Pat. No. 5,543,956 to Nakagawa et al. and U.S. Pat. No. 5,767,666 to Asada et al. are illustrative. An illustration of a galvanometric torsion oscillator is shown in FIG. 1. (The term galvanometric is believed to be a reference to coils on the turning member operated in the manner of a common galvanometer.)
The torsion oscillator of FIG. 1 comprises a central rectangular plate 1 suspended by two extensions 3a, 3b of the material of plate 1. Extensions, 3a, 3b are integral with a surrounding frame 5. Typically, the plate 1, extensions 3a, 3b and frame 5 are cut or etched from a single silicon wafer. A coil 7 of conductive wire and a region 9 of reflective mirror material are placed on the central plate.
This entire assembly is located inside a uniform magnetic field 11 (shown illustratively by lines with arrows), such as from opposing permanent magnets (not shown). When a current passes through coil 7, a force is exerted on coil 7 which is translated to plate 1 since coil 7 is attached to plate 1. This force causes rotation of plate 1 around extensions 3a, 3b which twist with reverse inherent torsion.
Other means may be employed to make such a system oscillate, such as static electricity or external magnetic fields. Various ones of such means are known in the prior art. The use of a coil drive by electric current in the embodiments disclosed herein should be considered illustrative and not limiting.
The spring rate of extensions 3a, 3b and the mass of plate 1 constitute a rotational spring-mass system with a specific resonant frequency. Plate 1 can be excited to oscillate at the resonant frequency with an alternating level passing through the coil and having a frequency at the resonate frequency or having some other frequency, such as harmonic at the resonate frequency. Where the input frequency varies from the resonant frequency and is substantial in power, plate 1 oscillates at the input frequency but drive level to coil 7 must be higher to achieve the same sweep (extent of oscillation) of plate 1. The device functions as a laser scanner when a laser is directed at the oscillating surface of mirror 9, thereby replacing the much bulkier rotating polygonal mirror widely used in laser printers and copiers. Torsion oscillators also have other applications, such as to drive a clocking device, in which mirror 9 would not be used.
The angle of mirror 9 moves sinusoidally with respect to time at a certain amount of sweep (termed amplitude), in a certain repetition rate (termed frequency), and with a potential lack of symmetry with respect to the using apparatus (termed median offset). These elements must be stabilized for useful operation. But the characteristics of a torsion oscillator can vary significantly from manufacturing tolerances and changing environmental conditions. Moreover, the direction of frequency drift is not readily determined since amplitude falls for drift to both higher and lower frequency. This invention provides two alternative control procedures which stabilize operation as the resonant frequency shifts during use.
In accordance with a first control procedure of this invention, drift is observed by sensing a reduction in amplitude. In response the original drive frequency is maintained and previous amplitude is restored by an increase in drive level and any undesired median offset is eliminated by an opposite change in the median of the drive level. This is the preferred control procedure where drift will not be so great as to overcome available power or power-use limits of the oscillator. This procedure is not preferred where the necessary level of power is impractical or the associated financial costs are too high.
In accordance with a second control procedure of this invention, the frequency of the drive signal to the torsion oscillator is set a small amount offset below or above resonate frequency. The direction of this frequency offset is known. Operation of the oscillator is observed to determine the amplitude of the oscillator (this may be inferred from the time the light of a scan beam activates a sensor twice). When the offset is below and the amplitude increases the drive frequency is reduced to stay below the new resonant frequency, when the offset is below and the amplitude decreases, the drive frequency is increased to remain close to the new resonant frequency. Similarly when an above offset is used and the amplitude increases, the drive frequency is increased to stay below the new resonant frequency. When an above offset is used and the amplitude decreases, the drive frequency is reduced to remain close to the new resonate frequency.
Operation of the device using the oscillator is necessarily at the power input frequency for both of the foregoing control procedures. Accordingly, operating frequency of the using device for the first control method remains fixed, while operating frequency for the second control method varies continually.